import math
a,b,e=input().strip().split(' ')
a=float(a)
b=float(b)
e=float(e)

def fun(x):
    if x==0:
        return 1
    else:
        return math.sin(x)/x


k=0.5
lst=[a, b]

h=b-a
x_list=[a+h/2]

T=0.5*h*(fun(a)+fun(b))

while True:
    h *= 0.5
    new_T=0.5*T+h*sum(fun(i) for i in x_list)
    k*=0.5



    if math.fabs(new_T-T)<e:
        print("%.6f" % new_T)
        break
    T=new_T


    new_x_list=[]
    for i in x_list:
        new_x_list.append(i-0.5*h)
        new_x_list.append(i+0.5*h)
    x_list=new_x_list



#while True:
    # new_list=[]
    # for i in range(len(list)-1):#2-> 0 1(not included)
    #     new_list.append((list[i]+list[i+1])/2)
    # new_T=0.5*T
    # #list=[i for i in range(a,b+(a-b)*k,(a-b)*k)]
    #
    # k*=0.5
    # for i in new_list:
    #     new_T+=k*fun(i)
    # if math.abs(new_T-T)<e:
    #     break


